This invention relates to a multicolor optical filter, and more particularly to a multicolor optical filter comprising an interference filter and a dye filter. The invention further relates to a process for making the multicolor optical filter.
Multicolor striped or mosaic optical filters are used for color solid image pickup elements, etc., typified by a color image pickup tube (or camera tube), a color charge coupled device (CCD), and a charge injection device (CID). A multicolor optical fiber usually comprises a regularly arranged stripe or mosaic pattern of red, green, and blue colors or cyan, magenta, and yellow colors. The color constitution of a multicolor optical filter is, however, not limited to the above-described three colors and other color arrangements such as two-color or four- or more-color arrangements are employed as the case may be.
Multicolor optical filters conventionally known are generally classified into interference filters and dye filters. An interference filter is composed of a transparent substrate such as a glass plate, etc., having formed thereon multilayer interference coatings for obtaining desired optical characeristics as described in Japanese Patent Publication Nos. 8590/'65; 27,742/'72; 23,247/'75; and 44,414/'75 and a dye filter is typically composed of a regularly arranged hydrophilic polymer layer dyed by dyes as described in Japanese Patent Application (OPI) Nos. 37,237/'72; 63,739/'73 and 66,853/'73, and Japanese Patent Publication No. 248/'78. An optical filter prepared by forming dye images in a silver halide emulsion layer by a coupler-in-developer type color development as will be described hereinafter belongs to the latter type.
Of these multicolor optical filters, an interference filter has particularly excellent optical characteristics but the step of making the filter of each color is very complicated and hence a multicolor optical filter obtained by repeating the complicated steps for two colors or three colors inevitably becomes expensive. On the other hand, the step of making a dye filter is simple as compared with the step of making an interference filter and, hence, a dye filter can be made at a relatively low cost. Furthermore, a multicolor optical filter can be obtained very easily by the process developed by some of the inventors hereof (as described in U.S. patent application Ser. No. 52,704, filed June 28, 1979 now U.S. Pat. No. 4,271,246) wherein a multicolor optical filter is prepared by light-exposing a black and white silver halide emulsion layer and developing it in a color developer containing a color coupler for each color and further according to this process, a pure black pattern called an optical black required for a multicolor optical filter for a color image pickup tube can be formed as a specific effect.
Consequently, the most preferred multicolor optical filter can be prepared by combining the advantage of an interference filter and the advantage of a dye filter.
Investigations in detail on dye filters along these multicolor optical filters have been made it has now been found that in comparing an interference type filter with a dye type filter, the dye type filter has inferior cyan or blue spectral characteristics.
FIG. 1 of the accompanying drawings shows a cyan spectral characteristic curve (A) of a typical interference filter and a cyan spectral characteristic curve (B) of a typical dye filter. A cyan dye filter has the disadvantages that the transmittance of the filter at the short wave length side (from 400 n.m. to 490 n.m.) and the long wave length side (from 500 n.m. to 600 n.m.) is lower than that of an interference filter and also fade-out of the cyan dye filter occurs. On the other hand, it has been found that the yellow spectral characteristics of a dye filter are the same as those of an interference filter.
Therefore, a stripe filter having good spectral characteristics can be obtained by combining a cyan interference filter and a yellow dye filter. Japanese Patent Publication No. 4129/'77 discloses the preparation of a one stripe filter uniting two sheets of stripe filters with each other but in this invention the same kind of interference filters are employed but the above-described technical idea of the advantage of an interference filter and the advantage of a dye filter when combined with each other is not disclosed.